Filters, screens, or screen inserts made of a dimensionally stable material, such as stainless steel plating, ceramic or another suitable, primarily hard material, are used in a variety of functional areas. Some specific examples include the separation of liquids and solids using various types of centrifuges, the drying of bulk solids using fluidized bed dryers, and the processing of sugar cane and sugar beet using sugar centrifugal. Filters, screens, or screen inserts for such processes are required to have attributes such as high overall open area, high mechanical stability, smooth surfaces, tapered openings, and high resistance to abrasion and other forms of wear.
In a known process for the production of a filter, screen, or screen insert, the cutting jet (usually a water jet filled with an abrasive) is guided so that strips remains between the two screen openings in the feed direction, whereby the feeding speed of the cutting head in accordance with the required strip length, which determines the distance between the two screen openings, is increased so that removal of the material is at least prevented enough so that the cutting jet does not penetrate the material.
This means that after cutting a screen opening, the cutting head is quickly accelerated for a brief period and then quickly slowed. The inertia imposed due to acceleration and braking has an extremely unfavourable effect on the stability of the cutting facility and on its oscillation characteristics, so that the requirements for optimised cutting quality are ultimately not achieved.
Of course, the feeding speed or acceleration must be viewed relatively, i.e. the cutting head can either be moved with a fixed blank or the blank with a fixed cutting head to insert the screen openings, whereby the fluid is guided through a nozzle positioned in the nozzle head under high pressure, such as about 3000 bar to about 4000 bar, as one example.
Another well-known configuration for the production of a screen insert makes use of switch valves to interrupt the cutting jet, in order to insert a hole or slit pattern in the blank, with which the supply of the pressurised fluid is interrupted in front of the cutting head or before entering the nozzle.
This naturally results in significant problems; especially with the respective switch frequency as required in the production of filigree patterns.
Due to the high pressure applied, the switch valve is subject to an exceptional load, so that the service life is very low. The resulting costs due to stoppage times to exchange the switch valve and procure it are an obstacle for optimised operations.
Above and beyond this, the quality of the screen openings has certain shortcomings because, with the known processes, especially when cutting slotted openings, the edges are shown to be frayed in the sense of a zigzag form.
With the high density of 5×103 to about 5×105 screen openings per square metre for certain applications, the relatively small width of the individual openings is reduced in some areas by the frayed edges which has a negative effect on the screening or filtering effect, because the defined outlet is practically not guaranteed.
WO 91/12930 A1 discloses a process for cutting materials with a cutting head in which the outlet channel flows into a fluid channel. However, the disclosed equipment that carries out the process is exclusively for applications at high environmental pressure, whereby, using the constant side supply of fluid during operation, abrasion to the outlet channel should be reduced and the jet shape of the cutting jet should be influenced. A functional interruption of the cutting jet is not possible with this construction.
This applies in the same way to equipment known from WO 2000/056466. In this case, the fluid jet supplied into the side of the outlet channel serves the exclusive purpose of changing the coherence of the cutting jet in order to be able to
satisfactorily cut materials with different characteristics, and especially materials with different values of hardness.
Furthermore, the known technology teaches that merely stopping a supply of abrasive while maintaining high water pressure is ineffective and results in significant scoring of the target material being cut, even subsequent to stopping or “turning off the supply of the abrasive. Thus, such methods are strictly taught away from by the body of art, as they are considered to produce a filter, screen, or screen insert having a weakened structure, and are taught to reduce control by an individual operating the cutting jet over starting positions for cutting.